The present invention relates to belt tensioners, and more particularly to belt tensioners for adjusting the tension, for example, on the timing belt reeved around a pulley on the crankshaft of the motor vehicle engine and a camshaft pulley.
Motor vehicle engines are provided with a timing belt for rotating the crankshaft and the camshaft in synchronism. Belt tensioners are developed for adjusting the tension on the timing belt.
Many belt tensioners of this type have been proposed which include a damping mechanism for avoiding resonance due to tensile vibration of the belt. Such known belt tensioners include one comprising a solid cylindrical fixed member to be fastened to the engine block or like stationary portion, a movable eccentric member rotatably provided around the fixed member eccentrically therewith and corresponding to a bearing inner ring, with an oil of high viscosity provided therebetween, an idler rotatably provided around the eccentric member and corresponding to a bearing outer ring, and a spring provided between the fixed member and the eccentric member for biasing the idler into pressing contact with the timing belt, the idler being movable about the fixed member to adjust the belt tension (see Unexamined Japanese Patent Publication SHO 63-47551).
With the belt tensioner described above, the viscous resistance of the high-viscosity oil serves to inhibit the resonance due to the tensile vibration of the belt.
The belt tensioner, which is disposed on the slack side of the belt, has the problem that when the belt slacks abruptly, the idler is unable to follow the belt since the idler moves slowly owing to the viscous resistance of the oil. Troubles are therefore likely to develop when the engine is rapidly accelerated or is brought to a halt. For example, when the engine is stopped, the crankshaft instantaneously rotates reversely, greatly increasing the tension on the slack side of the belt on which the tensioner is disposed. Consequently, it is likely that the idler will stop as moved greatly away from the belt by being pushed by the belt. The slack side of the belt rapidly slacks when the engine is subsequently started, whereas the idler, if at a stop as moved greatly away from the belt, fails to follow the rapidly slacking belt. As a result, the timing belt slips relative to the pulley, jumping over some teeth to result in an engine trouble.